The present invention relates generally to systems of dielectric ceramic compositions suitable for microwave applications and more particularly to systems of dielectric ceramic compositions suitable for use as dielectric resonators designed to be operated in a frequency range of microwaves for telecommunication applications.
Existing dielectric ceramic compositions suitable for microwave dielectrics are generally capable of being categorized into the following systems of compositions: (1) the BaO-TiO.sub.2 system; (2) the ZrO.sub.2 -SnO.sub.2 -TiO.sub.2 system; (3) the perovskite-type complex oxide system; (4) BaO-Nd.sub.2 O.sub.3 -TiO.sub.2 system; (5) BaO-Sm.sub.2 O.sub.3 -TiO.sub.2 system; and (6) the (CaSrBa) Zirconate system.
In Journal of Materials Science Vol. 6, pp. 1214-1226 (1971), W. Heywang discloses doped barium titanates having utility as dielectrics possessing an extremely high dielectric constant.
In Journal of The American Ceramic Society, Vol. 57, No. 10, pp. 450-453 (October 1974), H. M. O'Bryan, Jr. et al. disclose their investigations of the dielectric properties of ceramics in the TiO.sub.2 -rich region of the BaO-TiO.sub.2 system. In particular, the authors describe a new BaO-TiO.sub.2 compound, Ba.sub.2 Ti.sub.9 O.sub.20, in the composition range between BaTi.sub.4 O.sub.9 and TiO.sub.2. This new compound, which exhibits temperature-stable high permittivity and low microwave loss, is said to be obtained when calcining and sintering conditions are controlled carefully.
In Materials Research Bulletin, Vol. 16, No. 11, pp. 1455-1463 (1981), G. Wolfram et al. disclose the existence range, structural and dielectric properties of Zr.sub.x Ti.sub.y Sn.sub.z O.sub.4 ceramics wherein x+y+z=2. According to the authors, ceramic samples in the system ZrO.sub.2 -TiO.sub.2 -SnO.sub.2 were prepared in order to investigate the existence range of a homogeneous phase Zr.sub.x Ti.sub.y Sn.sub.z O.sub.4 with x+y+z=2. Lattice parameters, dielectric properties and thermal expansion were determined. A homogeneous solid solution phase was found in part of the compisition diagram. Its crystal structure is isomorphous with ZrTiO.sub.4. The unusual variation of the lattice parameters with the Sn content is discussed and the range of compositions suitable for dielectric resonators is defined.
In Journal of The American Ceramic Society, Vol. 67, C59-61 (1984). H. Tamura et al. disclose an improved high-Q dielectric resonator with a complex perovskite structure formed from Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 -BaZrO.sub.3. The authors also disclose that both sintering and crystallization of Ba(Zn,Ta)O.sub.3 -BaZrO.sub.3 were accelerated as compared to that for Ba(Zn,Ta)O.sub.3 alone and that the microwave Q value was also improved.
In "Microwave Dielectric Materials," Ceramic Dielectrics: Composition, Processing, and Properties edited by Hung Ling and Man Yan, Am. Ceramic Soc. (1988), K. Wakino et al. discuss the low temperature characteristics and the third harmonic distortion levels of microwave dielectrics in connection with the anharmonic terms in the crystal's Hamiltonian.
In U.S. Pat. No. 4,242,213, issued Dec. 30, 1980 with inventors H. Tamura et al., a system of dielectric ceramic compositions based on magnesium, calcium and rare earth metal titanates is disclosed. Specifically, there is disclosed a dielectric ceramic composition for microwave applications consisting essentially of a sintered mixture represented by the general formula: EQU (1-x)MgTiO.sub.3 -x(Ca.sub.1-y Me.sub.y)TiO.sub.3
wherein Me is at least one rare earth element selected from the group of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and wherein x and y are molar fractions of respective components and take values within the following respective ranges: 0.03.ltoreq.x.ltoreq.0.15, 0.001.ltoreq.y.ltoreq.0.06. The composition is said to be a fine-grained, dense ceramic with high permittivity, high quality factor and small temperature coefficient of resonance frequency.
One disadvantage associated with many of the systems of compositions described above is that they require substantial quantities of expensive elements, such as Sm, Sr, Ni, Co, and Zr.
Another disadvantage associated with many of the systems of compositions described above is that they are incapable of accommodating appreciable levels of impurities without incurring debilitating losses in dielectric properties.
Still another disadvantage associated with many of the systems of compositions described above is that they are rigidly dependent upon achieving specific stoichiometries which, in many instances, are difficult to obtain.
Still yet another disadvantage associated with many of the systems of compositions described above is that they do not include constituent compositions whose respective dielectric properties, such as temperature coefficient of resonant frequency and dielectric loss, differ substantially. As a result, such systems are frequently limited to use in a narrow range of environments wherein the dielectric properties of the constituent compositions are acceptable. This problem is illustrated by the situation in which the compositions of a given system are intended for use as microwave resonators. Where the constituent compositions of the system have similar dielectric properties, they are often only useful as microwave resonators when a single type of material, e.g., steel, nickel, or aluminum, is used to encapsulate the composition. On the other hand, it can readily be appreciated that if such a system were to include constituent compositions whose dielectric properties differed substantially, the range of suitable encapsulating materials would probably be increased as different compositions would be optimally suited for use with different types of encapsulating materials.